Labour has called on the government’s anti-sleaze watchdog to investigate a former Conservative cabinet minister, amid claims he mis-used his parliamentary office to carry out legal work.

The former attorney general has earned more than £800,000 for his work for law firm Withers, which is representing the British Virgin Islands government in a corruption case brought by the UK government.

According to The Times, Sir Geoffrey used his parliamentary office to undertake some of the work.

According to his register of interests, Sir Geoffrey did approximately 434 hours of work for Withers between January and July this year, at an average of more than 15 hours per week.

Angela Rayner MP, Labour’s deputy leader and shadow chancellor of the Duchy of Lancaster said: “This appears to be an egregious, brazen breach of the rules.

“A Conservative MP using a taxpayer-funded office in Parliament to work for a tax haven facing allegations of corruption is a slap in the face and an insult to British taxpayers.

“The Parliamentary Commissioner for Standards must investigate this, and the prime minister needs to explain why he has an MP in his parliamentary party that treats Parliament like a co-working space allowing him to get on with all of his other jobs instead of representing his constituents.

“You can be an MP serving your constituents or a barrister working for a tax haven – you can’t be both and Boris Johnson needs to make his mind up as to which one Geoffrey Cox will be.”

The MPs code of conduct states any facilities “provided from the public purse” are used “always in support of their parliamentary duties”, adding: “It should not confer any undue financial benefit on themselves”.

This is one of the rules that Standards Commissioner Kathryn Stone found that Owen Paterson broke before the Standards Committee recommended he be suspended from the House of Commons for 30 days.

Sir Geoffrey took part, by proxy, in Commons votes this year on the cladding scandal and on protecting the UK’s steel industry.

And, by taking advantage of the Commons allowing widespread proxy voting – introduced due to the COVID-19 pandemic – Sir Geoffrey was also able to appear at a corruption inquiry held in the British Virgin Islands, a British overseas territory, on the same day those votes were held.

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The revelation that Sir Geoffrey was voting remotely in the Commons while also taking part in lucrative legal work abroad comes amid a fresh focus on MPs’ second jobs following the Owen Paterson lobbying scandal.